Contemporary multi-channel radio frequency (RF) communication systems typically employ one or more channels to pass signalling and control information to subscriber units. Generally, each subscriber desiring to use the communication system must transmit a call request code and receive a channel grant code before any transmissions between subscriber units may commence.
Since the communication resources of RF communication systems are limited, it is known to limit the duration of any single transmission to a predetermined time interval. Should a subscriber attempt to transmit beyond this time interval, a warning tone precedes automatic preemption of the communication, and the communication channel is reassigned to other subscribers. While this procedure works well for full-duplex subscriber units, it is ineffective for half-duplex (or simplex) subscriber units because the system controller is not able to "time out" a transmitting half-duplex (or simplex) subscriber unit. That is, since the half-duplex (or simplex) subscriber units cannot simultaneously receive information while transmitting, they cannot receive the warning tone prior to call termination. Of course, the half-duplex (or simplex) transmissions could be interrupted without warning, however, this practice would likely confuse and frustrate the subscribers and impede effective communication. Accordingly, a need exists in the art to limit the transmission time of half-duplex (or simplex) subscriber units without disrupting the efficient operation of the communication system.